The present invention relates generally to a position encoder for encoding the position of a moving member, and relates more particularly to a magnetic sensor having magnetoresistive (MR) elements arranged to sense plural pieces of magnetic information recorded on an encoder wheel wherein each diameter wheel may contain a different number of pieces of magnetic information recorded thereon at an associated different spacing.
It is well known in the art that when a MR element made of a MR material such a permalloy is placed in a magnetic field and current is supplied to the element, the resistance value of the MR element varies depending upon the intensity of the magnetic field. Thus, when such a MR element is used in place of a magnetic head to sense magnetic information recorded on a magnetic recording medium, the recorded magnetic information can be reliably sensed even when the relative velocity between the magnetic recording medium and the MR element is zero, that is, even in a stopped condition of the magnetic recording medium.
It is also known in the art to use the principle of MR resistance value change due to the presence of a magnetic field to sense the velocity of rotation or angular position of rotation of a rotating body carrying magnetic information and moving in proximity to the MR element. A transducer apparatus using the principle of the detection of the resistance value of a MR element in the presence of a magnetic field to detect the relative displacement of a magnetic information carrying medium is disclosed, for example, in U.S. Pat. No. 4,039,936 issued Aug. 2, 1977 to A. L. Jones, et al., U.S. Pat. No. 4,403,187 issued Sept. 6, 1983 to Takahashi, et al., U.S. Pat. No. 4,628,259 issued Dec. 9, 1986 to Takahashi, et al., and U.S. Pat. No. 4,639,807 issued Jan. 27, 1987 to Sekizawa, et al.
It is also known in the art to record serial pieces of magnetic information on a circumferential track provided on the surface of a drum-like rotary magnetic recording medium with the MR elements of the sensor disposed to extend transverse to the circumferential direction of movement of the magnetic recording medium with the sensor being brought into close proximity and opposite to the magnetic information recorded on the magnetic recording medium. The serial pieces of magnetic information are generally recorded as north(N) and south(S) poles on the circumferential face surface of the recording medium with the spacing between the poles, also referred to as wavelength (.lambda.), being relatively small to achieve a high recording density and accordingly, to read the recorded information with a higher resolution during movement of the recording medium. Since only a finite number of poles may be recorded on a recording medium, such as an encoder wheel, a predetermined and generally limited number of pulses can be produced. It has been found that the resolution is improved by using a sensor having a plurality of MR elements arranged in a relatively phase shifted relation. That is, the MR elements are spaced apart from one another by a predetermined distance and in accordance with the spacing of the magnetic information recorded on the magnetic recording medium. As the spacing between each piece of recorded information on the magnetic recording medium is made smaller to increase the number of pieces of information and accordingly the number of output pulses to improve the resolution or sensor sensitivity, the spacing between successive MR elements must also be made smaller to accommodate the smaller spacing between each piece of recorded information. Therefore, a number of different sensors each having a different MR element spacing to accommodate a specific corresponding spacing of recorded information on a magnetic recording medium, such as, an encoder wheel, are required. Consequently it is necessary to keep a large inventory of different diameter encoder wheels of which each diameter may have a different number of pieces of recorded magnetic information and a corresponding different sensor for each of the different encoder wheels. Accordingly, it would be desirable to be able to use a single sensor to sense a number of different diameter encoder wheels each having a different number of pieces of magnetic information recorded at a number of predetermined distances from one another whereby the incremental movement of the encoder wheel can be detected with a high degree of accuracy.
In addition to sensing the incremental movement of an encoder wheel it is often desirable to be able to determine the absolute displacement of a moving member, for example, the number of rotations of a shaft of an electric motor or other apparatus to which the encoder wheel is coupled. Accordingly, it is also desirable to use the same magnetoresistive sensor to sense magnetic information recorded on the magnetic recording medium, such as, for example, on an index track of an encoder wheel wherein the magnetic information is recorded along an index track on the circumferential face surface of the wheel. A system for sensing magnetically recorded incremental and index information is disclosed in a copending application filed concurrently herewith and assigned to the same assignee as the present invention, and entitled MAGNETIC ROTARY ENCODER SYSTEM Ser. No. 117,036.
Another problem often encountered with MR sensors is due to thermal gradients that develop across the surface of a substrate upon which the magnetoresistive material is deposited to form the MR sensing elements. Heat generated when voltage is applied to the MR elements often causes the MR elements to shift from their aligned relationship thereby producing inaccurate sensing of magnetic information, no sensing or severely reduces signal amplitudes. Some MR sensors attempt to solve the thermal gradient problem by adding heating means to the substrate to maintain a constant temperature across the substrate surface. The solution is not acceptable because there is an increase in power consumption and heat dissipation and the sensor must be made larger to accommodate the heating means.
It is an object of the present invention therefore, to provide a single MR sensor having a MR element array that may be used with a number of different diameter encoder wheels of which each diameter wheel may have a different number of pieces of magnetic information recorded at corresponding different spacings from one another and along an incremental track on the circumferential face of the wheel.
It is a further object of the present invention to provide on the single MR sensor an additional array having MR elements arranged to sense magnetic information recorded on an index track on the circumferential face of an encoder wheel.
It is yet a further object of the present invention to provide a MR sensor that is substantially free of thermal gradients which affect the sensitivity and accuracy of the sensor.